1. Field of the Invention
The present invention relates generally to lighting control systems and, more particularly, to a photosensor and control system for switching or dimming lighting fixtures to reduce power consumption.
2. Description of the Related Art
Electric-powered lighting in commercial buildings in the United States accounts for 23% of the electricity consumed. Although the efficacy of the fluorescent lamp, the dominant electric-powered lighting source in the commercial sector, is unlikely to increase significantly during the next decade, there are significant opportunities to reduce energy consumption used for lighting in commercial applications. These opportunities are created by using daylight (or solar light) more effectively by controlling the amount of electric-powered light delivered in response to available daylight. Studies undertaken over the past 20 years have consistently shown that electric energy used to generate light in commercial buildings can be reduced by 10% -30% by using a photosensor to reduce the level of electric-powered light when daylight is available, and to maintain the electric light levels at design levels throughout lamp life. Consumers perceive daylight dimming systems as ineffective, however, and are reluctant to install lighting control systems that dim or switch electrical lighting fixtures when daylight is present.
Illumination control is difficult because the sensor, for practical reasons, is usually located on the ceiling or high on a wall, while xe2x80x9cusefulxe2x80x9d illumination is more closely associated with illumination of the task or work-plane (typically a desktop). Moreover, the ratio for the illumination level at the task location to the illumination level at the operational sensor location is different for solar light and electric light. This difference is due to multiple factors including room geometry and incident angles of the light source to the work surface.
A lighting control system employing a control algorithm that merely tries to maintain a constant sensor signal will not provide, in fact, adequate useful illumination as the distribution of light within the space changes to a higher composition of daylight. Task-to-ceiling illumination ratios typically vary by a factor of five or more when going from the conditions of 100% electric-powered lighting to 100% day light. Therefore, the sensor signal does not increase proportionally with the illumination of the task location. The typical outcome is that the electric-powered lights are dimmed too much in the presence of daylight. Occupants then complain of insufficient light and the control is disabled, or adjusted to allow very little dimming.
To overcome the problem of variable task-to-sensor light level ratios for solar and electric light, proportional control systems have been suggested. Proportional control systems require commissioning, however, which can be difficult and expensive thus limiting their effective use. Most products on the market do not offer sufficient adjustment capabilities (both in terms of adjustment mechanics and range of adjustment) to allow easy commissioning. Many photosensors must be moved to different locations using a trial-and-error approach to get satisfactory performance. Such movement is time consuming, aggravating, and expensive. For at least these reasons, commissioning is often not done completely or properly and the systems fail to work as intended.
The cost of installation and commissioning is another reason that consumers are reluctant to install lighting control systems to dim or switch electrical lighting fixtures when daylight is present. Current lighting control systems require the sensor, typically mounted on the ceiling, to be hard-wired between the lighting fixtures and their power source. Commissioning current photosensor lighting control systems typically requires the use of extraneous light meters and physical adjustment of the photosensor. Frequently commissioning must be performed during multiple daylight conditions, sometimes including measurements in the absence of daylight.
Another problem is that some users do not prefer the same level of illumination as the proposition of daylight to total available light changes. Preference studies have shown that, under some circumstances, people want higher levels of illumination as interior daylight levels increase.
An additional photosensor problem is that, when different sensors are used for commissioning measurements taken at different locations, they can have different sensitivity to infrared (IR) radiation. This difference affects system performance because daylight contains much more infrared radiation than fluorescent lighting for the same amount of visible light. Therefore, the photosensor dims the electric lighting when it is essentially sensing invisible IR radiation rather than visible light.
Significant effort has been directed to solving these problems as evidenced by patents and other references directed to proposed solutions. A summary of some of the more pertinent references follows.
U.S. Pat. No. 6,188,182 issued to Nickols et al. is directed to a power control apparatus with a digital processing mechanism which provides a signal for controlling power provided to electric lighting. The digital processing mechanism provides a first power level sufficient for lighting fixture start-up, and a second power level for reduced power consumption load corresponding to predetermined power levels for the particular time of day. The digital processing mechanism can provide (1) a second signal which is further responsive to a detected illumination level or a weighted average of detected illumination levels, as well as (2) manual inputs to change stored control parameters.
U.S. Pat. No. 5,701,058 issued to Roth is directed to a method of calibrating a dimmable lighting system. Illumination levels are measured under controlled indoor and outdoor lighting situations and a set point and gain are determined to maintain a constant lighting level at points of interest. A light sensor supplies a voltage signal to dimming electronics which calculate the amount of electric light needed using the set point and gain values.
An article by Rubenstein et al., xe2x80x9cImproving the Performance of Photo-Electrically Controlled Lighting Systems,xe2x80x9d J. of Illuminating Eng""g Soc. (Winter 1989), is directed to various control algorithms for dimming electrical lighting fixtures in response to an illumination detection signal. Closed-loop proportional control algorithms were found to outperform other lighting control algorithms tested.
Despite these efforts, photosensors are rarely used in commercial lighting applications in the United States. Consequently, a need exists for a photosensor and lighting control system that will be widely utilized to reduce power consumption by electrical lighting fixtures.
To meet this and other needs, and in view of its purposes, the present invention provides a lighting control system and photosensor that can provide a desirable level of lighting at a task location while saving energy. The system decreases the amount of controlled (typically electric) light in response to the presence of both uncontrolled (typically solar) light and controlled light, the difference in the ratios of an illumination level at a task location to an illumination level at an operational sensor location for uncontrolled light and controlled light, and a user""s lighting preference. In one embodiment of the present invention, the photosensor can self-commission to compensate for this difference in illumination ratios. As used in this document, xe2x80x9cself-commissioningxe2x80x9d means that the photosensor performs all measurements and all calculations required to determine the controlled light set-point and illumination level ratios for uncontrolled and controlled light in order to complete the commissioning procedure.
A responsive, closed-loop, proportional control algorithm is used in the present invention. The algorithm can allow for desktop illumination to increase slightly as uncontrolled (solar) light levels rise. The commissioning procedure is programmed into the photosensor to provide quick and easy commissioning. The photosensor performs the necessary measurements and calculations and prompts the operator to move the portion of the photosensor that senses illumination level.
The photosensor preferably includes a self-powered photocell unit having a photodiode and a wireless transmitter that transmits a wireless sensor signal responsive to the illumination level at the photocell unit. This arrangement allows the photocell unit to be easily moved for commissioning. In addition, all commissioning measurements can be taken using the same photodiode, reducing variability caused by differences in sensitivity to spectral differences between uncontrolled (solar) light and controlled (electric) light.
It is worth noting that a high-quality photosensor will enable xe2x80x9clumen maintenancexe2x80x9d dimming in addition to daylight dimming. The maximum light output of all lamps will decrease as they age. To accommodate this fact, lighting systems are currently designed to produce more than the required amount of light when they are new, so that the lighting system meets the design goal near the end of lamp life. By using a photosensor to control power to the lighting system, the input power to the lighting system can be decreased when is the lamps are new and gradually increased as the lamps age, saving a great deal of energy over the life of the system.
Energy savings resulting from the use of lighting controls, such as photosensors, vary depending on the application. In private offices, the energy savings can reach as high as 60% because lights can be turned off when daylight is available. Open-plan offices typically have lights on, however, although lights near perimeter windows can be dimmed, so savings are not as great. Research shows that current energy savings attributed to the use of existing light controls is 18% and that current market penetration of existing products is 3.3% or less of commercial or industrial floorspace. Using the present invention, the expected energy savings could go up to 30% and the market penetration could double to 6.6%.
With the documented energy savings, conventional photosensors could be currently saving 17.7 million kWh in the State of Connecticut. Consider the prospective advantages of the present invention. The combination of a reduction in energy consumption by 30% on average with a doubling of market penetration would result in a 59.1 million kWh savings annually in Connecticut alone. Using standard United States Department of Energy (USDOE) calculation methodology, the energy savings described above will result in an annual reduction of 45 million metric tons of CO2, 0.1 million metric tons of SO2, and 0.055 million metric tons of NOx in Connecticut. These reductions would be of great benefit to the air quality in Connecticut.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.